White Grubs in Florida Sugarcane
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White Grubs in Florida Sugarcane

   

White Grubs in Florida Sugarcane 1

R. H. Cherry and R. S. Lentini2

White grubs commonly found in Florida sugarcane fields are larvae (i.e., immatures) of beetles. These beetles are in the family Scarabaeidae also called "June beetles." The most common grubs in Florida sugarcane fields are in the genera Anomala, Cyclocephala, Euphoria, Ligyrus , and Phyllophaga . Of these grub pests, the species Ligyrus subtropicus is by far the most important pest. It is the largest of the grubs and causes the most damage. One study showed that L. subtropicus caused a 39% reduction in sugarcane yield in areas of high infestation. Because L. subtropicus is the white grub of primary economic importance, it is the species that is discussed in this document.

Biology

Ligyrus subtropicus completes its life cycle in one year. The life cycle consists of 4 stages - egg, 3 larval instars, pupa and adult. The adults are hard-shell, black beetles that are most active in late May and June. The final instar (large grub) may last for 8 to 9 months and is the most damaging to sugarcane. Mature larvae are 1.5 to 2" long with a head about 1/3" wide. Grubs have a characteristic "C" shape, 6 legs, and a white to cream colored body with a dark posterior ( Figure 1 and Figure 2 ).

Figure 1. Characteristic "C" shape of white grubs.
Figure 2. Larval, pupal, and adult stages of white grubs.

Damage

White grubs damage sugarcane by feeding on roots and underground stems. The first symptom is a yellowing (chlorosis) of the leaves. This is usually followed by stunted growth, dense browning, lodging, plant uprooting and death in heavily infested areas. Symptoms may be seen as early as September. Damage is usually more severe in ratoon crops and is most evident around the edges of a field. Grub damage is also worse on muck soils than sandier soils.

Ligyrus subtropicus infestation usually starts at the edge of a field and slowly spreads, in an irregular pattern, throughout the field. Infested fields may need to be replanted because ratoon regrowth and productivity can be severely reduced. Heavily infested areas may not be worth harvesting.

Control

Biological

Bacterial milky disease, fungal Metarhizium disease and parasitic nematodes occur naturally in sugarcane grubs in Florida.

The bacterium Bacillus popilliae causes "milky disease" in the larvae of Ligyrus subtropicus and Cyclocephala parallela ( Figure 3 ). It is called milky disease because of the white milky appearance of the grubs when infected with the bacteria. The bacterium is the most prevalent pathogen found in these grubs. The contribution of natural infection by milky disease in controlling grub populations is not known. Laboratory studies have shown C. parallela is more prone to infection than L. subtropicus .

Figure 3. White grub infected with milky disease (left; note whitish tip of adbomen) and healthy grub (right).
Metarhizium anisopliae is a fungus that can infest white grub larvae and kill them. The fungus is naturally present at very low levels in the soil in Florida sugarcane fields. Although the fungus has been shown to be highly pathogenic to the white grub larvae in laboratory studies, it is not a major mortality factor in the field. Additional research is needed to determine what factors are presently limiting Metarhizium disease in the field.

The use of nematodes as biological control agents for grubs has had mixed results. In a laboratory study, it was reported that L. subtropicus appeared to be an excellent host for the nematode, Steinernema feltiae, yielding 139,576 nematodes per grub. However, field studies on the control of L. subtropicus showed that field releases of S. feltiae did not cause a significant reduction in the number of L. subtropicus grubs in sugarcane. Field releases of Steinernema glaseri also failed to significantly reduce the number of grubs. It is yet to be demonstrated that nematodes may be used for grub control in Florida sugarcane.

Cultural Practices

Disking infested fields, reducing the number of ratoon crops, and flooding are methods of grub control in Florida. Disking kills many grubs and allows birds to kill many more. Freshly planted fields usually have little or no grub infestation

Although it is not always practical to flood, this control method can significantly reduce grub populations. The following points are to be considered if flooding is to be used to control L. subtropicus in sugarcane grown on muck soil.

  1. Positive identification of Ligyrus subtropicus should be made and the stage found (i.e, adult, egg, larvae) should be noted.

  2. Adults are essentially impossible to kill by flooding. Eggs are also very difficult to kill by flooding. These stages occur from approximately May through July.

  3. Larvae (grubs) and pupae, which occur mostly from August through April, are the easiest stages to kill by flooding.

  4. The warmer the weather, the better the flood will kill larvae and pupae. If the water temperature in the flooded field is 77 °F or higher, a continuous flood for 5 days will be sufficient for grub control.

  5. The flood water level should be about 2" above the soil surface. Many grubs will come to the soil surface and survive if there is less than 2" of standing water. Water depth greater than 2" will increase grub mortality very little, if at all.

  6. Given the option, the best time to kill grubs by flooding is in August. At this time, water temperatures are warm, rainfall abundant, and feeding damage by the grubs is just starting.

Chemical

Chemical control of white grubs has not been effective in Florida. Hence, there are no chemicals labeled for white grub control in Florida.

Footnotes

1. This document is ENY-664, one of a series of the Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed October 1991. Revised April 2002. This publication is also a part of the Florida Sugarcane Handbook, an electronic publication of the Agronomy Department. For more information you may contact the editor of the Sugarcane Handbook, R. A. Gilbert (ragilbert@gnv.ifas.ufl.edu). Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. R. H. Cherry, professor, and R. S. Lentini, coordinator computer applications, Everglades Research and Education Center, Belle Glade, Florida Agricultural Experiment Stations, Institute of Food and Agricultural Sciences, University of Florida. For more information you may contact: Dr. R. A. Gilbert (ragilbert@gnv.ifas.ufl.edu), Sugarcane Handbook editor, Everglades Research and Education Center, University of Florida, Belle Glade, FL, 33430. The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named and does not signify that they are approved to the exclusion of others of suitable composition.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

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